ActionScript 3.0 Cookbook: Chapter 1: ActionScript Basics
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Variables

Variables are convenient placeholders for data in your code, and you can name them anything you'd like, provided the name isn't already reserved by ActionScript and the name starts with a letter, underscore, or dollar sign (but not a number). The help files installed with Flex Builder 2 contain a list of reserved words. Variables are convenient for holding interim information, such as a sum of numbers, or to refer to something, such as a text field or sprite. Variables are declared with the var keyword the first time they are used in a script. You can assign a value to a variable using an equal sign (=), which is also known as the assignment operator. If a variable is declared outside a class method, it is a class variable. Class variables, or properties, can have access modifiers, public, private, protected, or internal. A private variable can only be accessed from within the class itself, whereas public variables can be accessed by objects of another class. Protected variables can be accessed from an instance of the class or an instance of any subclass, and internal variables can be accessed by any class within the same package. If no access modifier is specified, it defaults to internal.

Functions

Functions are blocks of code that do something. You can call or invoke a function (that is, execute it) by using its name. When a function is part of a class, it is referred to as a method of the class. Methods can use all the same modifiers as properties.

Scope

A variable's scope describes when and where the variable can be manipulated by the code in a movie. Scope defines a variable's life span and its accessibility to other blocks of code in a script. Scope determines how long a variable exists and from where in the code you can set or retrieve the variable's value. A function's scope determines where and when the function is accessible to other blocks of code. Recipe 1.13 deals with issues of scope.

Event handler

A handler is a function or method that is executed in response to some event such as a mouseclick, a keystroke, or the movement of the playhead in the timeline.

Objects and classes

An object is something you can manipulate programmatically in ActionScript, such as a sprite. There are other types of objects, such as those used to manipulate colors, dates, and text fields. Objects are instances of classes, which means that a class is a template for creating objects and an object is a particular instance of that class. If you get confused, think of it in biological terms: you can consider yourself an object (instance) that belongs to the general class known as humans.

Methods

A method is a function associated with an object that operates on the object. For example, a text field object's replaceSelectedText( ) method can be used to replace the selected text in the field.

Properties

A property is an attribute of an object, which can be read and/or set. For example, a sprite's horizontal location is specified by its x property, which can be both tested and set. On the other hand, a text field's length property, which indicates the number of characters in the field, can be tested but cannot be set directly (it can be affected indirectly, however, by adding or removing text from the field).

Statements

ActionScript commands are entered as a series of one or more statements. A statement might tell the playhead to jump to a particular frame, or it might change the size of a sprite. Most ActionScript statements are terminated with a semicolon (;). This book uses the terms statement and action interchangeably.

Comments

Comments are notes within code that are intended for other humans and ignored by Flash. In ActionScript, single-line comments begin with // and terminate automatically at the end of the current line. Multiline comments begin with /* and are terminated with */.

Interpreter

The ActionScript interpreter is that portion of the Flash Player that examines your code and attempts to understand and execute it. Following ActionScript's strict rules of grammar ensures that the interpreter can easily understand your code. If the interpreter encounters an error, it often fails silently, simply refusing to execute the code rather than generating a specific error message.

Don't worry if you don't understand all the specifics. You can use each recipe's solution without understanding the technical details, and this primer should help you understand the terminology.

See Also

Recipes 1.13 and 1.14

Section 1.4: How to Trace a Message

Problem

You need to trace out a message or the value of some data at runtime.

Solution

Use the trace function, pass the data to it, run your application, and look for a message in the Console in Eclipse.

Discussion

You can trace out a message, the value of a variable, or just about any other data using trace, just as you would in earlier versions of ActionScript. Some examples:

trace("Hello, world");

trace(userName);

trace("My name is " + userName + ".");

Since the .swf is now launched in an external browser, it might seem that there is no way to capture the output of these trace statements. Fortunately, it is possible, and this functionality has been built in to Flex Builder 2 via the Console view. The Console view is the equivalent of the Output panel in the Flash IDE. Although it is not open when you first start Eclipse, it appears when needed.

The only requirement to using trace and the Console view is that you use Debug to test your application. Doing so includes extra features in the .swf that allows it to communicate back to the Console behind the scenes and pass any messages you trace. The following class creates a variable, assigns a value to it, and then traces it, along with some other string data:

package {
    import flash.display.Sprite;
    
    public class ExampleApplication extends Sprite {
        public function ExampleApplication( ) {
            var userName:String = "Bill Smith";
            trace("My name is " + userName + ".");
        }
    }
}

Now when you debug your application, it launches as usual in your default browser. Close the browser and switch back to Eclipse. You will see that the Console view is now open and has displayed the data you traced out.

When you launch the debug version of an application, you must have the debug version of Flash Player installed. If you don't have the debug version of Flash Player, you'll see an error message notifying you, and you'll have to download and install it from http://www.adobe.com/support/flashplayer/downloads.html.

Additionally, the debug version of Flash Player can write trace content to a file. The file that Flash Player uses is determined by mm.cfg, a file that is stored in the following locations:

Operating system	Location
Windows XP	C:\\Documents and Settings\\[user name]\\mm.cfg
Windows 2000	C:\\mm.cfg
Mac OS X	MacHD:Library:Application Support:macromedia:mm.cfg

The mm.cfg file allows you to set the following variables:

TraceOutputFileEnable

The value can be 0 (don't write trace content to a file) or 1 (write to a file).

TraceOutputFileName

The path to the file to which to write. If a value isn't specified, then the content is written to flashlog.txt in the same directory as mm.cfg.

ErrorReportingEnable

The value can be 0 (don't write errors to the logfile) or 1 (write errors to the logfile). The default value is 0.

MaxWarnings

The maximum number of errors to write to the logfile. If this value is set to 0, there is no limit. If a larger value is specified, that limit is imposed and any errors beyond the limit are not written to the log.

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